1. Field of the Invention
This invention relates to a novel process and is more particularly concerned with improved processes for the preparation of polyimides, polyamides, and copolymeric mixtures thereof.
2. Description of the Prior Art
The reaction of diisocyanates with dicarboxylic acids and dianhydrides in solution to form polyamides and polyimides is well known in the polymer art; see for example U.S. Pat. No. 3,592,789 wherein there is disclosed the formation of coating solutions by reacting a diisocyanate, such as 4,4'-methylenebis(phenylisocyanate) with trimellitic anhydride in dimethylformamide at about 150.degree. F to 300.degree. F, and conversion to the cured polymer at 200.degree. C to 420.degree. C; U.S. Pat. No. 3,541,038 discloses the polymerization of trimellitic anhydride with diisocyanates at elevated temperatures; and U.S. Pat. No. 3,708,458 discloses the formation of polyimides requiring long reaction times. U.S. Pat. No. 3,701,756 teaches the use of certain hydroxides and ureas as catalysts for the reaction of isocyanates with anhydrides. However, such catalysts are extremely difficult to remove from the products so obtained. It is known to those skilled in the polymer art that the reaction of diisocyanates with dicarboxylic acids in solution to form polyamides requires even higher temperatures than those called for in the prior art hereinbefore cited; see for example U.S. Pat. No. 3,642,715.
It has been well established that isocyanates react with some dipolar aprotic solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and the like, at elevated temperatures. See M. R. Weiner, J. Org. Chem. 25, 2245 (1960) and S. Terney et al., J. Polym. Sci., Part A-1,8, 683 (1970). For example, heating of phenylisocyanate in dimethylformamide at only 150.degree. C for 150 minutes gives a 35% yield of N-phenyl-N',N'-dimethylformamidine and 30% of a cycloaddition adduct derived from a further reaction of the formamidine with four moles of phenylisocyanate. The side reactions arising during polymerizations involving the use of isocyanates in such solvents, have already been considered; see, The Reaction of Isocyanates with Polar Solvents, by H. Ulrich, paper presented at the University of Detroit, 1974 Polymer Conference Series. The side reactions easily lead to chain termination (i.e., lowering of polymer molecular weight), or crosslinking and incorporation of units other than amide or imide into the polymer chain, all of which are highly undesirable when high molecular weight, linear polymers are desired.
I have now found a process for carrying out the polymerization reactions hereinbefore described and known from the prior art, said process being free of the difficulties described hereinabove. The novel process of the present invention provides for lower polymerization temperatures, and shorter polymerization times, when compared to the prior art. As an added advantage to flow from the use of lower reaction temperatures, problems arising from possible solvent - isocyanate interaction have been eliminated. Therefore the soluble polymers obtained by the process of the present invention are characterized by having excellent molecular weight.